This invention relates to the correction of phase jitter which is experienced by digital data signals during transmission over media of limited frequency bandwidth. More particularly, this invention relates to phase jitter correction via the use of a multiharmonic adaptive phase compensator.
It has been well known in the art for some time to utilize limited frequency band channels for the transmission of data over telephone voice lines. Recently, the technology has advanced to the point where transmission of data at 14,400 and even 19,200 bits of data per second is accomplishable. In providing such high data rates it has been necessary to increase the number of points in the transmitted constellation while maintaining the same average power. With more constellation points, it has become advisable to correct for and/or eliminate as many impairments as possible because given the identical bit rate and amount of noise and impairments, fewer errors would be expected to occur in a system with fewer constellation points. In other words, the closer the constellation points are one to the other, the smaller is the transmission impairment which will cause an error.
In the modems of the art, it is well known that an incoming signal is demodulated and phase corrected. The so-corrected signal is then passed to circuitry which decides to which constellation point it corresponds. By comparing in a phase detector the complex signal being fed into the decision circuitry and the complex signal put out by the decision circuitry, a waveform representative of the phase may be obtained. The resulting phase waveform may then be fed back via a phase jitter canceller to the phase corrector which corrects the phase of the incoming signal.
It will be appreciated that the phase waveform resulting from the comparison of the complex signals may be said to take the form of: EQU .PSI.=.alpha. sin (w.sub.j t+.PHI.)+n(t) (1)
where .DELTA..alpha. is the amplitude of the phase deviation in degrees or radians, w.sub.j is the jitter frequency, .PHI. is the initial phase, and n(t) is the noise. In correcting for phase jitter, it is evident that it is desirable to drive the phase .PSI. to zero. Indeed, various approaches which attempt to accomplish this correction are known in the art. For example, in U.S. Pat. No. 4,320,526 to Gitlin, an adaptive phase jitter compensator seen in FIG. 1 is provided. The compensator 20 obtains signals which are sent to decision circuitry 22, conducts in phase detector 24 a phase comparison on the incoming signals with the complex signals output by the decision circuitry 22, and sends the determined phase error to the input 26 and the coefficient update 28 of an adaptive infinite impulse response filter 30 which is essentially comprised of a finite impulse response filter 32 with feedback 34. The IIR 30, in accord with techniques well known in the art automatically tunes itself to the predominant phase jitter frequency and provides a signal indicative of the predominant phase jitter frequency and amplitude of the phase jitter. The sine and cosine of the output signal of the IIR (itself a sine wave) are then taken at means 36 such that when the values are multiplied by the equalized signal, the complex equalized signal is corrected and the phase error minimized. Thus, the IIR provides a signal with automatically adjusted amplitude, phase, and frequency.
A second arrangement for phase jitter compensation is seen in FIG. 2 herein which substantially amounts to an equivalent arrangement of that described in U.S. Pat. No. 4,253,184 to Gitlin et al. In FIG. 2, it is seen that a digital oscillator 50 is provided and outputs the sine and cosine function for a given frequency and for second and third harmonics of the same. The sine and cosine functions for each harmonic are input into adaptive amplitude-phase correctors 52a, 52b, and 52c which also receive phase comparison information from a phase detector 54 which compares signals which are sent to decision circuitry 56 with the complex signals output by the decision circuitry 56. Given set frequencies w.sub.j, 2w.sub.j and 3w.sub.j, the adaptive correctors 52 adjust the amplitude and initial phase so as to minimize the output of phase detector 54.
While the adaptive phase jitter compensation arrangement of U.S. Pat. 4,320,526 does provide some phase correction, it will be appreciated that the provided arrangement assumes that all of the phase jitter is first order; i.e. that the phase jitter does not include any harmonics. When even small amounts of harmonics of the phase jitter frequency are present, however, the performance of the provided arrangement degrades significantly. On the other hand, while the phase jitter compensation arrangement of U.S. Pat. No. 4,253,184 does account for harmonics and permits the phase and amplitude of the jitter to be adaptively found, the arrangement assumes a particular frequency for the jitter. However, if the jitter frequency is not known in advance, the compensation technique of U.S. Pat. No. 4,253,184 cannot be implemented because it requires pretuning; i.e. it is not adaptive to jitter frequency.
Further, it should be appreciated that while a first arrangement provides a means for adapting to the frequency, amplitude and initial phase of the phase jitter, and a second arrangement provides a means for accounting for the phase jitter harmonics, the two arrangements cannot be easily combined. For example, the combination which would suggest itself would be to use additional adaptive IIR filters of U.S. Pat. No. 4,320,526 in the parallel arrangement of U.S. Pat. No. 4,253,184, with the phase error as decided by the phase detector as the input into each IIR filter in order to cancel the second and third harmonics. However, in providing such an arrangement, all three filters interact with each other and the entire system becomes unstable.